It is widely known that carbon dioxide is a green-house gas and considered one of the most important contributors to global warming. Many initiatives have been put forward by a great number of countries with the aim of reducing carbon dioxide emissions. The transformation of carbon dioxide into valuable chemicals, such as energy vectors like methane or methanol, is a highly desirable objective having received considerable interest in recent years. Most of the current systems capable of catalyzing the reduction of CO2 into valuable products, including notably the inverse water-gas shift reaction to generate carbon monoxide which in turn can be transformed into several useful chemicals, call upon the use of transition metals.[1-5]
Recently, some organometallic systems have shown promise in generating valuable chemicals in one pot under mild conditions. For example, Milstein described a ruthenium pincer complex to reduce CO2-derived carbamates, carbonates and formates to methanol using H2 as a hydrogen source.[6]
Methanol has also been obtained from CO2 and H2 by an elegant cascade reaction using ruthenium and scandium catalysts.[5b] The most active systems to date for the reduction of CO2 into high hydrogen content molecules include a ruthenium phosphine complex and a nickel pincer complex.[5c, 4a] An iridium catalyst has recently been described that can reduce CO2 into methane with a Turn-Over Number (TON) ranging up to 8300.[2]
Recently, a variety of transition metal-free systems have emerged for carbon dioxide activation. Lewis acidic Et2Al+ species were shown to catalytically reduce carbon dioxide to methane.[7b] Similarly, silyl cations were shown to catalytically reduce CO2 to a mixture of benzoic acid, formic acid and methanol.[8c] However, both systems greatly lack selectivity and generate undesirable alkylation by-products.
An interesting alternative for carbon dioxide activation is the use of “frustrated Lewis pairs” (FLP).[9a] Since its initial discovery, many ambiphilic systems have been shown to be active in the stoichiometric fixation of CO2.[9b-e] Piers demonstrated an elegant use of this concept for the catalytic reduction of CO2 into methane using the robust TMP/B(C6F5)3 (TMP=2,2,6,6-tetramethylpiperidine) system in association with Et3SiH, albeit with limited turnover numbers.[7a] The FLP system consisting of PMes3/AlX3, (X═Cl, Br) has also been shown to not only bind CO2 but also to reduce it to methanol by hydrolysis.[10a] O'Hare and Ashley also demonstrated that CO2 could be hydrogenated using TMP/B (C6F5)3.[10b] Unfortunately, these systems require stoichiometric amounts of FLP.
Some of the limitations in the CO2 activation by FLP systems include the generation of stable intermediates that limit the catalytic efficiency of the system as well as the deactivation of the catalytic system by the products generated. Although interesting in concept, none of the FLP or ambiphilic systems reported to date demonstrate efficient catalytic activity for carbon dioxide reduction.
The only efficient organocatalytic system reported to date for the reduction of CO2 into methanol uses highly Lewis basic N-heterocyclic carbene catalysts in combination with diphenylsilane as the hydrogen source with turn-over frequencies (TOF) of 25 h−1 at 25° C.[8a]
Ambiphilic systems with little “frustrated” character and/or weak Lewis acidity and basicity have recently been investigated.[9b] Among those, aryl bridged phosphine-boranes have been extensively studied by Bourissou.[11] These compounds were shown to be quite robust, stable and readily synthesized. Their use in the activation of singlet oxygen and as organocatalysts for Michael addition reactions has recently been disclosed.[12,13]
The present disclosure refers to a number of documents, the contents of which are herein incorporated by reference in their entirety.